The invention relates to the field of microelectronics and, more specifically, to the sector for fabricating microcomponents, especially those intended to be used in radio-frequency applications. More specifically, it relates to microcomponents such as microinductors or microtransformers equipped with a magnetic core allowing operation at particularly high frequencies.
As is known, electronic circuits used for radio-frequency applications, especially such as mobile telephony, comprise oscillating circuits including capacitors and inductors.
Given the trend towards miniaturization, it is essential that microcomponents such as microinductors occupy an increasingly small volume, while keeping a value of inductance which is high enough and a high quality coefficient.
Moreover, the general trend is towards increasing operating frequencies. Thus, mention may be made by way of example of the frequencies used in the new UMTS standards of mobile telephony, which are in the region of 2.4 gigahertz, in comparison with the frequencies of 900 and 1800 megahertz used for the GSM standard.
The increase in operating frequencies poses problems relating to the behaviour of magnetic cores of microinductors.
This is because, in order to obtain a good quality factor, an increase in the inductance of the microinductor is generally sought. To this end, magnetic materials are chosen, the geometry and dimensions of which enable the greatest possible permeability to be obtained.
However, given the phenomena of gyromagnetism, it is known that the permeability varies according to the frequency and, more specifically, that there is a resonant frequency beyond which an inductor has capacitative behaviour. In other words, a microinductor absolutely must be used at frequencies below this resonant frequency.
However, increasing the frequencies of use therefore comes up against the phenomenon of gyromagnetic resonance, which, for a given geometry, limits the frequency range in which the inductor can be used in an optimal manner.
A problem for which the invention proposes a solution is that of limiting the frequency of use inherent to the existence of a phenomenon of gyromagnetism.
The invention therefore relates to an inductive microcomponent, such as a microinductor or micro-transformer, comprising a metal winding having the shape of a solenoid, and a magnetic core including a strip made of a ferromagnetic material, positioned at the centre of the solenoid.
This microcomponent is characterized in that the core comprises at least one additional strip made of a ferromagnetic material, separated from the other strip by a spacer layer made of a non-magnetic material, the thickness of which is such that the strips located on either side of the spacer layer are antiferromagnetically coupled.
In other words, the two ferromagnetic strips of the core act one on the other through the spacer layer in such a way that the magnetizations of each of these strips are oriented in the same direction, but in opposite senses one with respect to the other. This antiferromagnetic coupling therefore opposes the rotation of magnetizations when an external field is applied. This resistance to the rotation of magnetizations results in a decrease of the intrinsic permeability of the ferromagnetic material taken in isolation. This is because, when the magnetization of a magnetic domain of one of the strips is subjected to an external magnetic field, the magnetization of the facing domain on the other strip interacts thereby limiting the rotation of the first magnetization.
The decrease in the magnetic permeability of each strip results in a decrease in the inductance of the microcomponents. It has been noticed that this drawback was compensated for by the increase in the gyromagnetic resonant frequency, corresponding to the maximum frequency at which the microcomponent retains its inductive behaviour.
As is known, the magnetic permeability is a complex quantity in which the real part represents the effective permeability (xcexcxe2x80x2), while the imaginary part (xcexcxe2x80x3) represents the losses. The gyromagnetic resonant frequency is given by the following expression:   fres  =            γ              2        ⁢        π              ⁢                            (                                    H              k                        +                                          N                ·                4                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              M                s                                              )                ⁢                  (                                    H              k                        +                                          4                ·                π                            ⁢                              xe2x80x83                            ⁢                              M                s                                              )                    
in which:
N is the demagnetizing field coefficient,
xcex3 is the gyromagnetic constant,
Hk is the value of the anisotropy field, and
Ms is the value of the magnetic moment at saturation.
It is therefore found that the resonant frequency increases with the value of the anisotropy field which orients the magnetic domains along the easy axis.
Thus, the presence of the second strip is comparable to increasing the value of the anisotropy field which characterizes the difficulty in imposing a rotation on the various magnetizations. The increase in the anisotropy field therefore implies, using the formula above, an increase in the resonant frequency. It is therefore possible to use the microinductor or the microtransformer according to the invention at higher frequencies than for the existing components.
In practice, the magnetic core may consist of a stack of a plurality of ferromagnetic strips separated by non-magnetic spacer layers, the strips located on either side of each spacer layer being antiferromagnetically coupled.
This is because, in order to obtain the antiferromagnetic coupling phenomenon, the spacer layer must be particularly thin, of the order of a nanometer, which requires a stack of more than about ten ferromagnetic layers in order to obtain inductances which are high enough.
In practice, each ferromagnetic strip has an easy magnetization axis which is preferably oriented perpendicularly to the axis of the solenoid. Such an orientation is obtained by applying a magnetic field perpendicular to the axis of the solenoid, while the material constituting the ferromagnetic strips is being deposited.
In practice, the spacer layer may be either a conductor, or even an isolator. From among the preferred conductors, copper, gold, silver, chromium and the metal alloys from these metals will especially be chosen. With regard to the insulators intended to form the spacer layers, silica, aluminium and silicon nitride may for example be used. The ferromagnetic materials used may be especially cobalt, iron, nickel-iron or cobalt-iron.